To manufacture plastic articles, plastic injection machines are used for plasticizing the supplied plastic granulates and injecting these with a high pressure into a mold installed into the plastic injection machine. The plastic cools off to a mold-release temperature in this generally tempered mold and can then be removed by ejector means. The working cycle of this machine, namely, the amount of ejection per unit of time is decisive for the feasibility of the plastic injection machine. To increase the speed of ejection, several cavities are as a rule provided, so that several parts can be manufactured simultaneously during the same cycle. It is known to provide several channels in the mold, which channels guide the plastic to the individual cavities. The disadvantage of this method, however, consists in the plastic provided in the channels also having to be ejected, thus creating a considerable waste, which in the case of small articles can amount to several 100% of the weight of the article.
To avoid such waste, it is known to insert heated channels between the clamping plate and the mold plate. These heated channels have nozzles and have the purpose of keeping the plastic ready for injection between the cycles and of limiting the waste. It is known to construct the heated channel in the of a block form and to insert heating elements into the channel with the heated channel having the same temperature inside and outside, namely, at the melting temperature of the plastic. This melting temperature may, for example in the case of polyamide 66 be 260.degree., while the mold temperature, for purposes of facilitating a quicker hardening of the plastic, is at approximately 60.degree.. The so caused disadvantages consist in the mold plates being able to be supported only through support segments, which results in an unstable mold. The longitudinal expansion between the heated channel and the other areas must thereby be considered simultaneously. Significant cooling is needed in order to keep the mold temperature low, which results in a waste of energy. Moreover, the heated channel also has a large mass and thus causes a great energy consumption.
Furthermore, heated channels are known in which the plastic provided in the bores is heated by heating wires placed into the bores. Surface heating of the plastic has the advantage that the temperature in the plastic is very even and thus very good injection results are achieved. The disadvantage, however, consists in the entire heated channel having to be heated up, resulting in the above-described problems. The heating of the channels in the heated channel from the inside out is more economical and requires less energy, however, the disadvantage exists here that the plastic cools off significantly toward its outer edges, so that in order for the plastic to have a sufficient injection temperature also at the edges, same must be significantly overheated in the center. This also reduces the injection cycles.
The basic purpose of the invention is to avoid the disadvantage of the conventional heated channels, in particular to provide a heated channel which can be connected flat to the clamping plate and mold plate without causing too great a heat transfer onto the clamping plate and mold plate, and which most importantly is easy and economical to manufacture.
According to the invention, the heated channel thus consists of a block in which is arranged a longitudinal bore into which terminate cross bores for the connection of the connecting nozzles and branching nozzles. An insert member with the material-guiding bores for the connection of the cross bores is inserted into the longitudinal bore. The insert member has grooves in its surface, into which grooves heating elements are inserted. Furthermore, connecting elements for facilitating a radial and an axial fixing of the heated-channel member and insert member are inserted in the heated-channel member.
The inventively constructed heated channel is thus constructed at least in two parts and consists of two separately manufacturable members, namely, the heated-channel member and the insert member. After the manufacture of the two members, they are moved into one another and are fixed in radial and also in axial direction by means of the connecting elements such that the material-guiding bores in the insert member are in alignment with the connections in the heated-channel member. The heated-channel member is constructed flat at its two surfaces which will rest on the clamping and the mold plate, so that a flat bearing is here obtained to thus provide a strong connection between the clamping and the mold plates. The insert member in turn does not need to transmit any pressure forces but must merely be constructed to connect the connecting nozzles and the material-guiding bores in the heated-channel member between the clamping plate and the mold plate. The heating elements are inserted into the grooves of the pipe-shaped constructed insert elements. Shielding sheet-metal plates can be placed onto the heating elements, which plates prevent or at least influence a radiation of the heat toward the heated-channel member. Furthermore, an insulating pipe can be moved between the heated-channel member and the insert element, which insulating pipe further reduces the heat flow between the insert element and heated-channel member.
A particularly advantageous and economically manufacturable construction of the heated-channel member is facilitated by the heated-channel member not only having a longitudinal bore, but also at least one bore extending transversely with respect to the longitudinal bore, into which bore nodal elements are inserted. The nodal elements have a corresponding longitudinal bore like the longitudinal bore of the heated-channel member and, moreover, have the connecting bores for supplying and discharging the material. Depending on the number of cavities formed in the mold and the chosen branching in the heated channel, two and more nodal elements can be inserted into the heated-channel member. The nodal elements are fixed in radial direction by the insert member moved therethrough and the heated-channel member, while the axial fixation is done by the connection elements which connect the nodal elements on the one hand to the insert member and on the other hand to the clamping and mold plate.
A significant advantage of an inventive heated channel consists, aside from its simple and strong construction, which permits the flat connection of clamping and mold plate, in the heat requirement being able to be reduced to 30 to 50% compared with otherwise commonly used heated channels which are heated from outside. This at the same time increases the injection cycle.